AMO/Marketplace currently use the HighCharts charting library from highcharts.com. However, this code is dual-licensed under either CC-BY-NC or a proprietary license, neither of which are open source.
http://shop.highsoft.com/highcharts.html
It is arguable whether our use of it counts as non-commercial; however, we did obtain permission from the developer, so that particular aspect is not a problem. We are not violating the license he has given us. However, the fact remains that the license is not open source, so we need to replace our use of this library with something which is. Tarek Ziade suggests Rickshaw and d3.js:
http://code.shutterstock.com/rickshaw/
Why do we need to do that? Our aim at Mozilla is to make great software that other people can use, for any purpose. This is certainly true of AMO/Marketplace, because we hope that there will be lots of Marketplaces, in contrast to the single Apple Store or Google Play.
However, people can't just take our code and use it to make a new marketplace if some of it has a "no commercial use" restriction. That's why we write open source software, and why the goodness of doing so is written in our manifesto.
Related bug: 758185 (input.mozilla.org).
Gerv

I suggest Flot because it is the most fully featured, canvas-based FOSS graphing library I know. Rickshaw has matured quite a bit since I last looked; it also seems like a viable replacement now. I'm a little concerned about SVG performance on mobile, but maybe that has also improved since I looked.

Resolving this bug as WONTFIX suggests that this situation is acceptable. Without wanting to lay blame on anyone for how we got to where we are, this isn't acceptable.
If we are serious about the FxOS ecosystem being different, and being very cool with multiple marketplaces existing, then we should be removing barriers to other people building marketplaces. Making our marketplace properly open source (and therefore usable by anyone, including commercial entities) is a good step there.
Gerv